import java.util.Scanner;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main2 {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);

            int n= in.nextInt();
            int k = in.nextInt();
            int []a=new int[n];
            int count=0;
            int maxc=0;
            for (int i=0;i<n;i++){
                a[i]=in.nextInt();
            }

            for (int i=0;i<n;i++){
                for (int j=i;j<=i;j++){
                    if (Math.abs(a[i]-a[j])<=3){
                        count++;
                        if (maxc<count){
                            maxc=count;
                        }
                    }
                }
            }
        System.out.println(count);;
        }
    }
